Ofcom says it will not use its research to force mobile phone operators to improve services in rural areas. Photograph: Alamy

One in five calls made by Vodafone customers in rural areas are failing, according to official figures by the telecoms regulator which lay bare the poor service suffered by customers in some parts of the country.

Research by Ofcom found that while overall levels of consumer satisfaction with mobile networks are high – 76% report themselves as happy – only two thirds of those living in rural areas say they enjoy a good mobile service.

Ofcom used independent firm RootMetrics to analyse call data, and found that typically only around of 85% of calls made in rural areas pass off without a problem.

It found that just 79.9% of Vodafone 2G and 3G calls in rural areas completed successfully, compared to 86% over the 3 network, 87.4% on O2 and 93.7% on EE. In urban areas, EE was again top, successfully completing 97.5% of calls, with Vodafone again bottom at 95.3%. The figures include "blocked" calls, when a user has a signal but can't make a call due to lack of capacity, and "dropped" calls, when a call is connected but terminates unexpectedly. The issue is not just frustrating for users but also costly, as they have to call two or three times to complete a conversation.

In June, David Cameron – a Vodafone customer – ordered ministers to improve mobile phone coverage across the countryside after becoming frustrated at the lack of mobile reception. He revealed that while on holiday in Cornwall he has had to drive to the top of a hill to find a connection to speak to other world leaders.

Operators have been reluctant to build extra masts in the countryside for relatively few users, provoking calls for the network to be nationalised.

Ofcom says these latest figures present a truer picture than data from the mobile operators, which measure the proportion of completed calls, but excludes call attempts when customers are outside of network coverage. This, in part, is likely to explain why the call failure rates reported by RootMetrics are higher than those from the mobile operators, it says.

An Ofcom spokesman said the report was designed to aid consumers in their choice of provider. But it will not be used by the regulator to make the phone companies improve services in rural areas, which will further anger those living in the countryside. Under the terms of their licences the mobile operators have to meet the 90% coverage obligations for 3G mobile.

Ofcom says it will be publishing research comparing 3G and 4G mobile broadband speeds later this year, following detailed testing across five UK cities.

"This will help consumers choose a service that suits them and encourages providers to improve performance," it says.

A spokeswoman for Vodafone says: "The previously published RootMetric report is based on network measurements taken between June and December last year which were disproved by more up-to-date findings in spring this year."